1. Field of the Invention
The invention relates to a process for continuous coating deposition and an apparatus for carrying out the process. The invention more particularly relates to a process for forming oxide based ceramic coatings on reactive metal and alloy sheets, foils and wires that are in the form of a web in a continuous manner and an apparatus therefor. The films obtained according to the present invention have a glossy surface finish, thermal and electrical insulation, chemical inertness, environmental inertness, surface cleaning ability, anti-dust sticking and have good scratch resistance. Further, the process described in the present invention deposits the oxide ceramic films at a rapid rate and enhances the productivity to a great extent.
2. Description of Related Art
The metals like Al, Ti, Mg and their alloys are commercially and widely used in the engineering industries like automobile, aerospace, textile, petrochemical and crockery in the form of rods, bars, tubes, foils, sheets, wires, pipes, channels, sections, pulleys, cylinders, pistons, etc. Apart from the specific promising properties and commercial availability that these materials have, the main reason for using these materials is their high strength to weight ratio. However, there exists a limitation to use these materials beyond a certain point; the limitation arises from the fact that these materials exhibit poor resistance to wear and tear, chemical attack and heat.
Traditionally, anodizing is employed to obtain coatings on Al-alloys. But the resultant coatings are found to be porous and weakly adherent to the substrate, and thereby can not provide high level protection against wear and tear and corrosion. Moreover, coating deposition rates achieved are also low in the anodizing process.
Thermal spraying techniques like plasma spraying, high velocity oxy-fuel spraying, and detonation spraying are well developed and widely used by the engineering industry to produce large varieties of metallic, oxide, carbide and nitride based ceramic coatings. These coatings are essentially employed to combat various forms of wear and tear and corrosion and thereby enhance the service life of the components made of different metals and alloys. However, thermal spray techniques demand a high degree of pre-coating and post-coating operations that are often costly. Size, shape and complexity in geometry of the engineering components do restrict the applicability of the thermal spray techniques. Moreover, these techniques demand high quality as well as costly powders such as Alumina, Alumina-Titania, Tungsten Carbide-Cobalt, Chromium Carbide-Nickel Chrome prepared by specially developed manufacturing routes such as sol-gel, atomization, fusing, sintering and crushing, chemical reduction and blending. Deposition efficiency of these powders is always much less than 100% thus requiring a special means of unused powder separation from the coating chamber. Since these coating techniques employ spraying of heated powder particles on to relatively cold surfaces, poor metallurgical bonding between the substrate and the coating often results. These coatings are often characterized by inherent porosity, micro-cracks and higher levels of residual stresses which in turn lead to the failure of the coatings in the case of critical applications. Due to the associated coating deposition mechanism, the thermal spray techniques are not at all suitable to deposit thin films on sheets, foils and wires. Moreover, it is not practically possible to deposit thin coatings on thin sheets, foils and wires in a continuous manner.
Yet another field of research in the area of thin film deposition on sheets, foils and wires is by means of Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) techniques. However, due to the inherent nature of these processes wherein the overall coating deposition is significantly influenced by the ionic/atomic scale interactions with the surfaces being coated, the overall coating deposition rates are extremely low and production rates are very low. Besides the slow deposition nature of these processes, these techniques are also not suitable for coating deposition on a continuous scale on extremely larger/longer surface areas.
To overcome the above-mentioned difficulties and limitations and the present day need for coatings exhibiting improved tribological, electrical, thermal and chemical properties and having higher density and excellent wear resistance, research work in the area of developing an improved micro-arc oxidation process has gained importance globally.
There exist a good number of patents and publications which deal with ceramic coating deposition processes on aluminum and its alloys. Some relevant literature on prior art micro-arc processes is referred to below.
According to U.S. Pat. No. 6,197,178, a three-phase pure sinusoidal potential of 480V AC electrical power is supplied to aluminum alloy web and current densities between 20 and 70 A/dm2 are applied. During the process, current density is maintained by moving the web relative to each other. An electrolyte with KOH, Na2SiO3 and Na2O.Al2O3.3H2O in the proportion of 2 grams per liter of de-ionized water is used. That temperature of the electrolytic bath is maintained between 25° C. and 80° C. The coating thickness achieved is reported to be in the range of 100 to 160 microns for a 30 minute processing time on cylindrical samples.
Although the resultant coatings were found to have strong adherence with the substrate, no information is available with respect to the density and uniformity of the coatings achieved. Coating density is a very important parameter that affects the wear resistance of the resulting coatings.
In the invention cited above, the inventors used a pure sinusoidal voltage waveform without any waveform modification, while a sharply-peaked waveform makes a major contribution in providing a dense and hard coating. This is why the coatings obtained through the above-mentioned process exhibit lower hardness, i.e., 1200-1400 kg/mm2. However, there is no mention of the application of the said process to deposit coatings on thin sheets, foils and wires or to do so in a continuous manner.
U.S. Pat. No. 5,616,229 granted to Samsonov et al. discloses a method of forming a ceramic coating on valve metals. This method comprises application of at least 700V alternating current across the parts to be coated. Waveform modification is achieved through a capacitor bank connected in series between a high voltage source and the metallic body to be coated. Waveform of the electric current rises from zero to its maximum height and falls to below 40% of its maximum height within less than a quarter of a full alternating cycle.
The electrolyte used in the above cited process contains 0.5 grams/liter NaOH, 0.5-2 grams/liter KOH. In addition, the electrolyte also contains sodium tetrasilicate for which there is no claim on the exact amount to be added. During the process, the electrolyte composition is changed by adding oxyacid salt of an alkali metal in a concentration range of 2 to 200 grams per liter of solution. The process has been demonstrated by coating an aluminum alloy known as Duralumin by employing three different electrolytic baths. However, in the process explained above there is no mention of maintaining any particular ratio between the alkali and metal silicate.
In the micro-arc oxidation process, alkali is actually responsible for dissolving the coating, whereas the metal silicate is responsible for coating buildup through poly condensation of silicate anions. Too high silicate concentration in the electrolyte causes higher coating buildup especially at sample edges rather than at other portions of the sample thus resulting in a non-uniform coating. Hence, there is a need to maintain a certain degree of proportion between the alkali and metal silicate in order to end up with uniform and dense coatings. However, there is no mention of the application of the said process to deposit coatings on thin sheets, foils and wires or to do so in a continuous manner.
In the process disclosed in U.S. Pat. No. 5,616,229 a process is described wherein an average deposition rate of 2.5 micron per minute has been achieved. However, the thickness of a fully melted inner layer is only 65 microns out of a total coating thickness of 100 microns. This indicates that this process can produce coatings comprising only 65% initial dense layer and remaining 35% external layer is porous with 4-6 pores per sq. cm. and an average pore diameter of 8-11 microns.
To make these coatings suitable for wear resistant applications, the external porous layer of sufficient thickness needs to be completely removed by machining or grinding. Apart from the fact that these machining or grinding operations are costly, machining/grinding of coated parts of complex, non-symmetric shapes is extremely difficult and demands a high degree of automated machinery and higher skill levels. This effectively increases the cost of the coating per unit volume. However, there is no mention of the application of the said process to deposit coatings on thin sheets, foils and wires or to do so in a continuous manner.
The prior art processes of micro-arc oxidation processes yielded thick dense, adherent coatings with higher coating deposition rates but failed to produce thin films on a continuous scale so as to coat several meters and kilometers long sheets or foils and wires wherein it is essentially required to impart a glossy surface finish, thermal and electrical insulation, chemical inertness, surface cleaning ability, environmental inertness, anti-dust sticking and have good scratch resistance to find potential applications in the field of decorative, insulation, anti-dust sticking applications.
Moreover, in the prior art, the processes employed for coating metallic web has been discussed in detail, but nothing has been disclosed about the general apparatus employed for carrying out the coatings on thin sheets, foils and wires or to do so in a continuous manner process in continuous scale.
According to the invention disclosed in U.S. Pat. No. 6,197,178, the apparatus employed for obtaining the coating consists of a chemically inert coating tank disposed within an outer tank. The outer tank contains heat exchange fluid. Electrolyte from the inner tank is circulated through the heat exchange disposed in the outer tank itself. To remove heat from the heat exchange fluid, heat exchange fluid is withdrawn from the outer tank with the help of a pump and then passed through a forced air cooled heat exchanger. The operation of the exchangers was controlled automatically so as to maintain the desired temperature within the electrolyte bath. However, there exists a serious drawback with this kind of setup. When a component of larger size than that of the inner coating tank is to be coated, the dimensions of the inner tank must be increased, which in turn may demand changing the outer tank dimensions as well. This makes the process more costly.
In our Indian Patent No. 209817, the following process has been described:
A process for forming coatings on bodies of reactive metals and alloys which comprises electrolysing in a non-metallic, non-reactive, non-conductive reaction chamber containing an alkaline electrolytic solution having a pH>12 and conductivity >2 millimhos, comprising potassium hydroxide, sodium tetrasilicate and de-ionized or distilled water, immersing at least two metallic bodies selected from the reactive group of metals on which coatings have to be effected, the bodies being fixed in a movable manner, each body being connected to an electrode, passing wave multiphase alternating current across the said bodies by thyristors connected in parallel for a period based on the desired thickness of the coating to be achieved, slowly increasing the current being supplied to the said bodies until the required current density is achieved, then maintaining the current at the same level throughout the process, the electric potential being further increased gradually to compensate for the increasing resistance of the coating when the visible arcing at the surface of the immersed regions of the said bodies is noticed, regulating the composition of the electrolyte by measuring its pH and conductivity during the process by conventional methods, maintaining the temperature of the electrolyte between the range of 4° C. to 50° C. and keeping the electrolyte in continuous circulation throughout the process.
The apparatus for carrying out the process as disclosed in the patent comprises a non-metallic, non-conductive, non-reactive chamber (1) (named as reaction chamber) housing at least two metallic bodies (2), the surfaces of which are to be coated, the bodies being connected to the electrical power carrying arm (3) provided with a height adjustable mechanism (4), an inlet (5) for the electrolyte provided at the bottom, and an outlet (6) at the top of the chamber, on the panel of main controller (8), analog voltmeter (9), and ammeter (10) being provided to indicate the input voltage and current, a lever type electric power on/off (11) being provided, a potentiometer (12) provided for slowly increasing the current supply to the metallic bodies (2), contactor on/off (13), thyristor on/off (14) switches, manual/automatic voltage adjustment (15), and local/remote operation (16) selector switches being also provided, thyristor (not shown) and transformer (17) outputs being connected through the separate analog voltmeters (18) and ammeters (19), two separate digital temperature indicators (20) being attached to the panel of remote controller (21), the temperature of electrolyte at the inlet and outlet being measured through the thermocouples (not shown), an oscilloscope (22) attached to the remote controller (21) for monitoring the electrical potential and current waveforms during the process, digital voltmeter (23) and ammeter (24) attached to the remote control panel (21) being used to monitor the changes in the current and voltage during the coating process, the height of electrolytic column (7) in the reaction chamber (1) being adjusted through a dimmerstat (25) attached to the panel of remote controller (21) and an emergency stop button (26) being attached to the remote control panel (21) for terminating the electrical power supply to the bodies in the case of any emergency.
The drawbacks of the apparatus disclosed in our earlier Indian Patent No. 209817 are listed below:
1. the apparatus is not suitable for depositing thinner coatings on large area surfaces;
2. the apparatus is not suitable for depositing coatings on thin foils, sheets and wires;
3. the apparatus is suitable for depositing thicker coatings (85 to 95 microns as illustrated in Example 1 and Example 2 described in Indian Patent No. 209817) that possesses quite rough surface finish. Thereby the surface cleaning ability is poor and prone to dust accumulation;
4. the apparatus is not suitable for production scale as it is merely batch type processing based on the design of the electrolytic bath and also by the way that the bodies to be coated are arranged in the bath, which consumes a lot of time for fixing the bodies to be coated; and
5. the apparatus works with only two-phase electrical energy and leaves the third phase unutilized, therefore leading to electrical imbalance in the electrical mains.
Hence, it can be seen that there exists a need for providing a process for depositing uniform, thin films on sheets, foils and wires so as to enhance surface finish, thermal and electrical insulation, chemical inertness, surface cleaning ability, anti-dust sticking and to have good scratch resistance as well depositing in a continuous manner and also an apparatus for carrying out the process.